Light curable composition finds many applications in paints, coating, optical and microelectronic adhesives, dental adhesive and composites et al. UV and visible light are two common light sources to promote such curing reaction. Furthermore such a light curing process can also be classified as photo-induced cationic polymerization or photo-induced free radical polymerization based on the nature of photo-initiator and mechanism of polymerization. For any given light curable composition there is at least composed of a curable resin and photo-initiator. Typical curable resin include (meth)acrylate, vinyl, vinylether, epoxy, et al. Photo-initiators are photo-sensitizer, such as arylketone, diketone, acylphosphine oxide et al. Besides additional coinitiator or accelerator is also required in order to produce an effective curing. It has been found that tertiary aromatic amines, such as Ethyl-4-DimethylAmine Benzoate (EDAB), are the most effective accelerators. However, the accelerating efficiency between these tertiary aromatic amines depends heavily upon the substitution on the aromatic moiety. Moreover there is increasing concerns over the potential toxicity and the long-term effect on color stability for the resulting cured systems based on those conventional tertiary aromatic amines. It was believed color stability was influenced more by the nature of the aromatic ring than by the substituent on the nitrogen atom. Discoloration of the cured matrix containing such a tertiary aromatic amine is the increasing concern, especially when it is used as dental restoratives where color match is much more critical. Therefore there is a need for readily obtained, reactive, and color stable amine as novel photo coinitiator.
For dental application, it is more desirable to use photoinitiator which is color stable but demonstrate high ambient light (environmental light) stable and promote quick polymerization later on. Such a photoinitiator system composing an α-diketone, an aliphatic/aromatic amine and a triazane derivative, was disclosed in US Paten Application 2004/0180983. However, it is concerned about its potential problems in color stability and shrinkage stress concentration associated with for such a initiator system due to the nature of the isomerization nature of the triazine compound and the fast cure reaction rate.
One reason behind color shifting of cured dental resin and composite is attributed to the residual amine, which is generally a tertiary aromatic amine. UV or thermal aging can cause its isomerization to form colored structure. One solution to disrupt possible conjugation through such structural isomerization of aromatic moiety by making a substitute aromatic and isolated such aromatic ring. 2,2′, 6,6′-tetrasubstitute biphenyl have been explored to make colorless polymers.
Another issue associated with free radical-based light curing process is the shrinkage and shrinkage stress due to the fast curing nature of conventional free radical photopolymerization. Therefore there is need to develop new photo-initiator which can demonstrate good balance between quicker curing rate and lower curing stress for a novel free radical curing system. It is the intention for this invention is to further tune down the radical polymerization rate in an effort to reduce the shrinkage stress for a given cured composition.
It is well known that the origin of stress in adhesive resin composite restorations is due to the restrained shrinkage during polymerization and it is dependent on the configuration of the restoration. Moreover, non-homogeneous deformations during functional loading can damage the interface as well as the coherence of the material. Damage from these stresses can be reduced by application of elastic lining at the adhesive interfaces and by slowing the initial conversion by two-step light initiation of the resin. The various factors that mediate flow and compliance are all have something to do with polymerization stress buildup or failure of a restored tooth. In addition to the nature of resin composition, how a given resin or composite is cured is also critical to the total stress development, which means a kinetic control on the polymerization stress development is possible. With increasing MW, polymer chain mobility was limited, the diffusion become the rate control factor. In a comparison with linear system, the limited mobility in a cross-linking system appear to come earlier, which means extra reaction would lead to an increasing polymerization stress. Although such a cross-linking reaction could not allowed scarifying to exchange a low stress because it did contribute the mechanical property to the final material.
There are different approaches to control the stress generation and development:    1. Slow down the polymerization rate;            Introducing a special rate controller like stable radicals or P&P resin system developed recently in this Company;        Creating different polymerization zones from which the stress developed in a polymerized zone could be transferred to its adjacent unpolymerized zone and got relief like segmental polymerization technique developed in this company (U.S. Pat. No. 6,783,810);        Employing different polymerization groups such as hybrid monomer with (meth)acrylate and vinyl ether.        Using large-size macromonomer to limited its reactivity at the early stage;            2. Reduce the conversion;            Pre-building a 2D or 3D structure like polymerizable macrocyclics developed in this Company, or dendremers or hyperbranches            3. Further tuning radical polymerization kinetic so as to allow stress relief during the course of cross-linking formation.
The present invention relates to a dental composite composition based on a novel tertiary aryl amine, more specifically a multiple, tertiary aryl amine which allow a better control over photopolymerization kinetics so as to generate much less curing contraction stress within a normal cross-linked system. In addition, composition from such novel amine can also offer improved color stability as comparison to that based on a conventional mono-tertiary aryl amine such as EDBA. Obviously the benefit from such a concept would not limit in dental composite or other dental materials. The application can be found in optical coating, microelectronic, et al.